Calculation of automated friction clutch urge torque on grades

ABSTRACT

The invention relates to a method for calculating a modification to the predetermined amount of torque needed to provide an urge to move sensation for a vehicle, the modification to take account of the weight at which and gradient on which the vehicle is operating and then commanding the engine to generate said modified torque and to transmit the modified urge torque to a clutch device to provide the urge to move according to the current operating conditions.

FIELD OF INVENTION

This invention relates to improvements in vehicle transmissions, inparticular to vehicles fitted with automated transmissions and clutchsystems.

BACKGROUND

When driving a vehicle fitted with a change gear ratio transmissionconnected to an engine by a torque converter the operation of thetransmission is characterised by the slip and torque multiplicationavailable from the torque converter section. The “feel” of thetransmission is recognisable by an “urge to move” when the vehicle isstationary. This urge to move is caused by the stall torque of thetorque converter at engine idle speed. On an uphill incline it canprevent the vehicle moving backward when the vehicle is stationary andthe driver moves his foot from the brake to the accelerator pedal.

An Automated Mechanical Transmission (AMT) includes a change geartransmission and normally also includes a Central Processing Unit (CPU)which controls the selection of gear and the operation of the gearselector mechanism. Normally, an AMT is coupled to a conventionalfriction clutch, rather than a known hydraulic torque converter. In moreadvanced versions of the AMT, the clutch control is also automated andthe driver has only two pedals in the cab, a throttle control oraccelerator pedal and a brake pedal. Normal or manually operatedclutches control the clutch and do not present the driver with an “urgeto move” feeling. When stationary, the clutch is normally completelydisengaged in order to minimise the clutch wear and avoid excessive heatdissipation that would arise from a continually slipping clutch. Inthese conditions there is nothing to prevent a vehicle fitted with anAMT rolling forwards or backwards if the driver has not applied eitherthe parking brake or service brake using the pedal.

In a commercial vehicle having 6, 9 or even more forward speed ratiosthere is the possibility of selecting a gear other than first gear forengagement. Commonly in such vehicles with a 6 speed transmission thestarting gear will be 2^(nd) or 3^(rd). For heavier vehicles with 12 or16 speed transmissions the starting gear will be 3^(rd) or 5^(th) andoccasionally 7^(th) if the vehicle is unladen and setting off in adownhill direction. With the choice of a number of start ratios it ispossible to select a ratio which can minimise the clutch slip and heatgeneration during the launch phase.

It is desirable to present the driver with an indication the vehicle isready for launch and can move off quickly when commanded to do so. Inthis specification the expression “urge to move” is intended to meanthat a vehicle is ready to move and the drive line slack has been takenup and there is a sensation given to the driver that the vehicle willmove off if the brakes are released. It does not necessarily mean thatthe vehicle will creep forward (or backwards), as would tend to be thecase, for a vehicle fitted with a conventional torque converter, but itis intended to indicate that sufficient torque is being transmittedthrough the vehicle drive-line to take up any slack in the drive-linecomponents and that the vehicle will not just roll in a downhilldirection if it is at rest on a gradient without a brake applied. TheUrge Torque is the amount of torque required to give the urge to movefeeling and make the vehicle feel as if it is ready to move off fromrest. It is an empirically determined figure that will depend on theweight and type of vehicle. It can be qualitatively described as theamount of torque needed to provide the driver with the sensation that atleast any slack in the drive-line has been partially taken up and theclutch engagement process has started and so the vehicle is ready tomove off.

Allowing the partial engagement of the clutch to provide slip under zerospeed conditions partially replicates the sensations of a torqueconverter type system and provides the urge to move feeling experiencedwith such systems. An automated clutch system can be programmed totransmit a greater amount of torque and so provide a limited amount ofmovement and so replicate the “creep” function. Alternatively, when avehicle is on an uphill gradient it is desirable to provide a hillholding capability in which the vehicle can be held stationary by theengine running at or slightly above idle speed and slipping clutchwithout the need for application of an additional braking force. It isalso clear that when a vehicle is laden the amount of torque required toprevent the vehicle rolling backwards is greater than that required whenit is unladen or on a flat or level surface. Clearly, it is desirable toensure enough torque is available to provide the urge to move in theparticular circumstances which the vehicle minimise the amount of slipand therefore heat generated in order to maximise clutch life.

SUMMARY

According to the present invention there is provided an automatedfriction clutch adapted to be fitted to a vehicle and engageable totransfer power from an engine to a mechanical change gear transmissionand a central processing unit for controlling operation of the clutch,and capable of sending command signals to the engine and transmission, aplurality of sensors, whose outputs are supplied as inputs to thecentral processing unit (CPU) for use in calculating the gradient onwhich the vehicle is operating and the weight of the vehicle, thecalculated values of gradient and vehicle weight being used by thecentral processing unit to calculate a value of modified urge torque andto provide command signals to the engine to deliver to the clutch atorque output equal to the modified urge torque and to a clutch operatorto engage to said clutch to the transmit the modified urge torque.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings in which:

FIG. 1 shows a general schematic view of a transmission system, and

FIG. 2 shows a flow chart for the operation of a logic sequence foroperation of a transmission in the mode described.

DETAILED DESCRIPTION

FIG. 1 shows a transmission system 10 includes an engine E having anoutput shaft 12 connected to a clutch C, which is in turn connectable toan input shaft 16 of a change gear transmission 11. The transmission 11is has an output shaft 20 connected to the drive wheels (not shown) ofthe vehicle.

The transmission system is controlled by a central processing unit (CPU)30, which preferably is a single processing unit, but alternativelycould be a plurality of processing units distributed processing units.In such circumstances the processing units may be located on thetransmission, in the vehicle cab, on the engine, on the chassis or anycombination of these. The transmission will normally have a number ofmodes in which it can operate, including manual and automatic. There maybe additional functions to enable the shift points to be adjusted tosuit the prevailing conditions.

The engine power demanded by a driver is signaled by THL 22, whoseoutput signal is sent to the CPU 30. The CPU 30 then communicates thedemanded power as an output signal to the engine E along link 23. Thedriver will also be provided with a gear ratio selector lever 34, usableto select a transmission ratio or to override the selection made by theCPU if the transmission is in automatic mode. Operation of the clutch iscontrolled by the CPU whose control signals are sent to a clutchoperator 27. Operation of the transmission will be by known means notforming a part of this invention. The gear ratio selector lever 34operates a set of contacts in unit 36 to provide an output signal to theCPU 30. The selector lever 34 is used by the driver to select a gearratio or to override the ratio selected by the transmission CPU. Furtherinputs to the CPU are from sensors ES, IS and OS which measure enginespeed, transmission input shaft speed and transmission output shaftspeed respectively. Output shaft speed can be used to determine vehiclespeed in known manner. The transmission controller 29 will also supplyinformation about the currently engaged gear ratio (GR). From thisinformation, it is possible to calculate GVW in known manner. Anothersource of GVW information could be the SAE J1939 data-link or “CAN bus”system if one of these is fitted to the vehicle.

FIG. 2 shows a flow chart sequence for implementation of the presentinvention. Values of throttle position THL, output shaft speed OS,engine speed ES and input shaft speed IS are taken from the sensors 22,25, 28, and 30 and supplied to the CPU which processes the informationto determine if the vehicle is on an up grade, level or a down grade andalso to determine the vehicle weight

The information on grade and weight is then used to calculate the amountof torque (modified urge torque) needed to maintain the vehicle in asteady state in the particular circumstances. Urge torque is normallydefined to be a specific value of torque which is commanded from theengine to provide the driver with a feeling of the urge to move, that isto say the drive line slack has been taken up and the vehicle is readyfor launch. It is normally a specific value of torque for a particulartype of vehicle and has been determined empirically. A medium dutyvehicle operating up to say about 26 tonnes, (approx 50,000 lbs), anurge torque of about 61 Nm (approx 45 ftlbs) has been determined to be agood value. Factors affecting the precise value of urge torque are thosesuch as vehicle configuration, and whether it is preferred for thevehicle launch smoothly and with a minimum of delay or whether a slowerresponse time is acceptable.

The CPU will calculate the value of modified urge torque, UT_(MOD),required to hold the vehicle in the particular circumstances. If thevehicle is on a level surface the urge torque will be equal to thenormally stored and predetermined value. If the vehicle is facinguphill, then more torque will be needed to hold the vehicle in positionwithout it rolling backwards.

Whilst the normal urge to move torque will slow the rate at which thevehicle rolls backwards compared to a similar set of circumstances whenthe vehicle is on the level, it is desirable, if possible, to provide inaddition, a hill holding capability. In providing a full hill holdingcapability a number of important criteria need to be considered,primarily the thermal performance of the clutch and its ability todissipate the heat generated during the hill holding process. Clearly, asmall capacity clutch will have a lower thermal capacity (and torquetransmitting capacity) than a comparable larger unit, and for similarsized clutches, wet clutches fitted with coolers have more capacity thanordinary wet clutches, which in turn have more capacity than dryclutches. In determining whether a system can have a full hill holdingcapacity these factors have to be considered. It will be important alsoto incorporate thermal warning and thermal protection systems, thedesign and details of which fall outside the scope of this invention.

If the vehicle is on a downhill gradient, then it will be desirable toreduce the modified urge torque so that it is less than the urge torque.The reduced value has a number of advantages, one of which is to reduceclutch slip and clutch wear. Also, if the vehicle is facing downhill, itis desirable to have the urge to move feeling, but with a reducedstrength so that the vehicle is not accelerated down the hill, unlessthe driver wishes or commands such acceleration by pressing thethrottle.

It can thus be seen that the modified urge torque is not always going tobe greater than the pre-determined urge torque, but in the downhillcondition it could be less. Although its value may be low it cannot benegative, and will not be zero if the slack in the drive train is to betaken up.

In order for the engine to generate the required torque it is necessaryto raise the engine speed above idle level, primarily to ensure thecontrol of the engine rests with the transmission CPU rather than theengine CPU. An increase above engine idle speed of between 50 and 100rpm is needed depending upon the engine, but normally about 50 rpm issufficient to achieve the desired effect.

The CPU of an automated transmission can normally calculate from inputsensor information, such as transmission input shaft speed, output shaftspeed, gear ratio and vehicle acceleration/deceleration the gradient ofthe road and the gross vehicle weight (GVW). Alternatively, suchinformation may be available from an SAE J1939 data link or CAN-bus ifit is fitted to the vehicle. Using the values of gradient and GVW, andwith information about the torque characteristics of the engine it ispossible to calculate the torque required to hold the vehicle inposition on an uphill gradient

The CPU also receives an input of gear ratio engaged which is used todetermine if the transmission is in one of the start gears. Sensedthrottle position (THL) provides an input 22 which is compared against apre-determined reference value THL_(REF) and the output shaft speed isalso compared against a pre-determined reference value OS_(REF). If thethrottle setting is greater than the pre-determined reference value, andif output shaft speed is greater than the pre-determined referencevalue, then the CPU will determine the vehicle is not at rest or idleand so discontinue the routine and exit the process.

If the throttle setting is less than the pre-determined reference value,and if output shaft speed is less than a pre-determined reference valueand the vehicle is in one of the permitted start gear ratios, then theprocess will continue. Providing the test conditions are satisfied theCPU can command that the engine speed be raised above idle speed toprovide the modified urge torque. Once the engine is generating themodified urge torque the CPU commands the clutch controller 27 to startengaging the clutch to transmit the modified urge torque. The routinethen ends.

This routine can be used to optimise the amount of clutch slip for allconditions. The benefits of this system are a consistent feel to the“urge to move” independent of the load or grade. By generating only therequired amount of torque it is possible to keep the clutch slip to theminimum required whilst still presenting the driver with the “urge tomove” feeling. Minimising the amount of torque generated by the enginewill minimise the amount of torque to be transmitted by the clutch andso reduce the amount of wear. This has the advantage of reducing theamount of heat dissipated in the clutch. This enables the thermalcapacity of the clutch to be used more efficiently. In the case when thevehicle is on the level or an up incline, this could mean a longerperiod of clutch slip, and in the hill holding mode a capability to holdthe vehicle on a steeper gradient. These benefits can be used tomaximise the clutch life or improve the capability of the system underthese difficult conditions.

1. An automated friction clutch (C), adapted to be fitted to a vehicleand engageable to transfer power from an engine (E) to a mechanicalchange gear transmission (11) and a central processing unit (CPU) forcontrolling operation of the clutch, and capable of sending commandsignals to the engine (E) and transmission, a plurality of sensors (22,25, 28, 30) whose outputs are supplied as inputs to the centralprocessing unit (CPU) for use in calculating a gradient on which thevehicle is operating and a weight of the vehicle (GVW), the calculatedvalues of gradient and vehicle weight being used by the centralprocessing unit (CPU) to calculate a value of modified urge torque(UT.sub.MOD) and to provide command signals to the engine (E) to deliverto the clutch (C) a torque output equal to the modified urge torque(UT.sub.MOD) and to a clutch operator (27) to engage to said clutch (C)to the transmit the modified urge torque (UT.sub.MOD).
 2. A systemaccording to claim 1 in which the modified urge torque is calculated tobe equal to that required to maintain the vehicle in a stationaryposition on a level or an up gradient.
 3. A system according to claim 1in which the modified urge torque is equal to or more than the urgetorque if the vehicle is on a level or an up gradient.
 4. An automatedfriction clutch (C), adapted to be fitted to a vehicle and engageable totransfer power from an engine (E) to a mechanical change geartransmission (11) and a central processing unit (CPU) for controllingoperation of the clutch, and capable of sending command signals to theengine (E) and transmission, a plurality of sensors (22, 25, 28, 30)whose outputs are supplied as inputs to the central processing unit(CPU) for use in calculating a gradient on which the vehicle isoperating and a weight of the vehicle (GVW), the calculated values ofgradient and vehicle weight being used by the central processing unit(CPU) to calculate a value of modified urge torque (UT.sub.MOD) and toprovide command signals to the engine (E) to deliver to the clutch (C) atorque output equal to the modified urge torque (UT.sub.MOD) and to aclutch operator (27) to engage to said clutch (C) to the transmit themodified urge torque (UT.sub.MOD); whereby the modified urge torque isless than the urge torque if the vehicle is on a down gradient.
 5. Amethod of controlling an automated friction clutch system coupledbetween an engine (E) controlled by a throttle and a mechanical changegear transmission (11), said clutch system connected to a centralprocessing unit (CPU), and in which the CPU: i) receives an input oftransmission gear ratio engaged (GR), ii) determines if the gear ratioengaged is a start gear, iii) receives a sensed throttle position (THL)input (22) from a throttle position sensor which senses throttleposition of said engine E, iv) compares sensed throttle position (THL)against a pre-determined reference value (THL.sub.REF) and v) comparestransmission output shaft speed (OS) against a pre-determined referencevalue (OS.sub.REF), and if the transmission is in one of the allowedstart gear ratios, and if the throttle setting (THL) is less than thepre-determined reference value(THL.sub.REF), and if output shaft speed(OS) is less than a pre-determined reference value (OS.sub.REF), thencalculates a value for a modified urge torque (UT.sub.MOD) and commandsthat the engine speed be raised above engine idle speed to providemodified urge torque (UT.sub.MOD) and commands a clutch operator (27) todeliver the modified urge torque to the transmission (11).
 6. A methodaccording to claim 5 in which the modified urge torque is calculated tobe equal to that required to maintain the vehicle in a stationaryposition on a level or an up gradient.
 7. A method according to claim 5in which the modified urge torque is equal to or more than the urgetorque if the vehicle is on a level or an up gradient.
 8. A method ofcontrolling an automated friction clutch system coupled between anengine (E) controlled by a throttle and a mechanical change geartransmission (11), said clutch system connected to a central processingunit (CPU), and in which the CPU: i) receives an input of transmissiongear ratio engaged (GR), ii) determines if the gear ratio engaged is astart gear, iii) receives a sensed throttle position (THL) input (22)from a throttle position sensor which senses throttle position of saidengine E, iv) compares sensed throttle position (THL) against apre-determined reference value (THL.sub.REF) and v) comparestransmission output shaft speed (OS) against a pre-determined referencevalue (OS.sub.REF), and if the transmission is in one of the allowedstart gear ratios, and if the throttle setting (THL) is less than thepre-determined reference value(THL.sub.REF), and if output shaft speed(OS) is less than a pre-determined reference value (OS.sub.REF), thencalculates a value for a modified urge torque (UT.sub.MOD) and commandsthat the engine speed be raised above engine idle speed to providemodified urge torque (UT.sub.MOD) and commands a clutch operator (27) todeliver the modified urge torque to the transmission (11); whereby themodified urge torque is less than the urge torque if the vehicle is on adown gradient.
 9. An automated clutch comprising: a mechanical changegear transmission; a central processing unit adapted to controloperation of a clutch disposed within said transmission and capable ofsending command signals to an engine and said transmission; a pluralityof sensors whose outputs are supplied as inputs to said centralprocessing for calculating a value of modified urge torque; and wherebysaid clutch delivers a torque output equal to said modified urge torque,and wherein the modified urge torque is calculated to be about equal tothat required to maintain the vehicle in a stationary position on alevel or an up gradient.
 10. The automated clutch of claim 9, whereinthe modified urge torque is less than the urge torque if the vehicle ison a down gradient.
 11. A method of controlling an automated clutchsystem comprising the steps of: providing a mechanical change geartransmission; adapting a central processing unit to control operation ofa clutch disposed within said transmission and capable of sendingcommand signals to an engine and said transmission; calculating a valueof modified urge torque by having a plurality of sensors whose outputsare used as inputs to said central processing; and delivering a torqueoutput at said clutch equal to said modified urge torque, wherein saidmodified urge torque is calculated to be about equal to that required tomaintain the vehicle in a stationary position on a level or an upgradient.
 12. The method of controlling an automated clutch system 11,wherein said modified urge torque is less than the urge torque if thevehicle is on a down gradient.